Thorp report calls for sustained excellence

26 February 2007

UK safety authorities have completed their analysis of the internal leak of radioactive liquor at Sellafield's Thorp facility. It draws lessons for all companies operating potentially hazardous plants, both inside and outside the nuclear industry.

Although the leak at Thorp was contained by design and did not put workers or the public at risk, the Health and Safety Executive (HSE) has concluded that the failure to promptly detect it was down to the "inadequate monitoring arrangements and management oversight" of operator British Nuclear Group Sellafield Limited (BNGSL).

In his foreword, Mike Weightman, the Chief Inspector of Nuclear Installations, called for sustained excellence in nuclear operation: "High standards are expected of the nuclear industry. This means continued vigilance and close attention to maintaining each and every one of the multiple physical and administrative barriers put in place to protect people and society from highly radioactive material."

"It is not acceptable to let any of ther barriers degrade or weaken, relying on the existence of other barriers to secure continued protection."

Among the HSE's conclusions for industry were:

All nuclear workers need to understand the key precautions necessary to ensure nuclear safety. This "reduces the likelihood that short cuts and workarounds will emerge over time."

A questioning attitude towards potential safety issues is essential. Leadership on this must come from senior management, who cannot rely on the absence of incidents as an indicator that everything is as it should be.

Design changes, even those that are apparently minor, must be appropriately assessed by those that understand their safety significance in relation to the original design intent of a facility.

Organisations must learn from their experiences. A structured, rigorous system is necessary to implement corrective actions and it must be followed up to completion.

He concluded: "HSE will ensure that the lessons learnt from this event are applied right across the nuclear industry and also, where relevant, for other major hazard industries. The public have a right to expect no less."

In April 2005 it was discovered that a pipe carrying highly radioactive dissolver product liquor had ruptured inside the UK's Thermal Oxide Reprocessing Plant (Thorp). Over a period of months, a body of the liquor had slowly built up, contained on the floor of a heavily shielded cell. In January 2005, the rate of leakage increased as the pipe was finally severed. The problem was revealed in April that year after accountancy calculations on the plant performance revealed discrepancies between materials input and output.

The Health and Safety Executive (HSE) report found that 83,000 litres of liquor leaked from primary containment to secondary (from the pipe itself to the steel-lined concrete cell). The liquor contained 22,000 kg of uranium and around 160 kg of plutonium, but "there was no possibility of a criticality." Shortly after the problem was discovered, existing arrangements were used to pump the material back into primary containment.

The leak was caused by inconsistencies in the later stages of design and construction, followed by a change in operating practice which led the pipe to undergo stresses it was not designed to take. After 11 years of operation, the pipe began to fail.

Although the leak itself did not put workers or the public at risk, the HSE concluded that operator British Nuclear Group Sellafield Limited's failure to promptly detect it was down to "inadequate monitoring arrangements and management oversight."

The HSE's Nuclear Installations Inspectorate made its own investigation, from which came 55 recommendations for improvement at Sellafield as well as two Improvement Notices for BNGSL. The investigation identified that BNGSL had made three serious breaches of its nuclear site licence which continued over a long period of time. "These breaches amounted to serious offences." The company was charged; it pleaded guilty and paid a £500,000 fine, plus £68,000 costs.

Thorp has been shut down since the discovery of the leak while BNGSL developed a new operating procedure that would not need the damaged pipe, which is very difficult to repair. BNGSL were given permission to restart Thorp on 9 January 2007.